Transistor with emitter at bottom of groove extending crosswise the base



lufi 1, 3965 NELSON T L 3,187,241

TRANSISTOR WITH EMITTER AT BOTTOM OF GROOVE EXTENDING CROSSWISE THE BASE Original Filed March 27, 1957 INVENTOR. #56554? NEASfl/V 6 I 9 La /510M; M. M

United States Patent 3,187,241 TRANSISTOR W11 ll EMITTER AT BOTTOM OF GROOVE EXTENDING CROSSWISE THE BASE Herbert Nelson, Princeton, N.J., and John Bernath, Los Angeles, Calif., assignors to Radio Corporation of America, a corporation of Delaware Original application ldar. 27, 1957, Ser. No. 648,855, now Patent No. 3,022,568, dated Feb. 27, 1962. Divided and this application Jan. 3, 1962, Ser. No. 163,992 7 Claims. (Cl. 317-234) This application is a division of application Serial No. 648,855, filed March 27, 1957, issued on February 27, 1962 as U.S. Patent 3,022,568.

This invention relates to improved semiconductor devices, more particularly, to improved broad-area junction devices such as transistors.

It is known to make broad-area junctions by immersing a semiconductive Wafer of one conductivity type in an ambient or vapor of .a material that diffuses into the wafer and induces conductivity of the opposite type. A surface layer is thus formed having conductivity type opposite to that of the bulk of the Wafer. The interface or junction between the surface layer and the bulk of the Wafer constitutes a rectifying barrier. The wafer requires subsequent processing to form the desired device. In such processing it has been found diflicult to control the size and particularly the exact shape of the diifusion-doped regions of the units by conventional methods, such as masking and etching the wafers in acids. Another serious problem that arises from the small sizes of the wafer and the shallow depth of the opposite conductivity type surface layer is the difiiculty of making good electrical connections to the different zones of each unit. Techniques for this purpose which are satisfactory in the laboratory are often not suitable for mass production.

It is therefore an object of this invention to provide improved semiconductor devices.

Another object of this invention is to provide improved broad-area junction transistors.

A further object of this invention is to provide improved methods of controlling the size and exact shape of the diffusion-doped regions of broad-area junction devices.

Still another object of this invention is to provide improved broad-area junction devices having a structure which facilitates the making of electrical connections to the several regions thereof.

These and other objects of the invention are accomplished by the utilization of a precision multiple lapping technique instead of etching to fabricate improved semiconductor devices having broad-area junctions of controlled size and shape. For example, recessed area of controlled size and shape are formed on a major surface of a given conductivity type water by lapping regular grooves across one major wafer face. A surface layer of the wafer is converted to opposite conductivity type by diflfusing an appropriate impurity into the wafer. The wafer is lapped again so as to remove the opposite type surface layer from the grooved face, excepting the recessed areas. A coating of conductive metal is deposited over the entire wafer, and it is then lapped again so as to widen the upper portion of the grooves. Thereafter the wafer is diced into units which are fabricated into devices such as transistors by conventional techniques for mounting, attaching electrical leads, and encapsulating.

The invention will be described in greater detail with reference to the accompanying drawing, in which:

FIGURE 1 is a schematic isometric view of the first step in making a junction device according to one embodiment of the invention, in which the collector region is the original wafer.

FIGURES 2-8 are schematic cross-sectional views illustrating successive steps in the fabrication of the device.

FIGURE 9 is a schematic isometric view of the unit after the operations shown in FIGURES 1-8.

FIGURE 10 is a schematic isometric view of the lapping jig used in the method of this invention.

FIGURES 11 and 12 are isometric views of lapheads used in the method of this invention.

FIGURES 1317 are schematic cross-sectional views illustrating the fabrication of a transistor according to another embodiment of the invention, in which the original Wafer is used as the base region.

FIGURE 18 is a schematic isometric view of the structure made by the operations shown in FIGURES 13-17.

Similar reference numerals are applied to similar elements throughout the drawing.

Referring to FIGURE 1 of the drawing, a wafer 20 of monocrystalline semiconductive material of given conductivity type is prepared. In a typical example the wafer is N-conductivity type silicon of about 05-10 ohm centimeters resistivity. The exact size of the wafer is not material. For example, the wafer may be one inch long by one half inch wide by eight mils thick. Regular grooves 21 are lapped into one major face of the Wafer 20, so as to produce recessed areas across the wafer. In this example, the grooves 21. are about mils Wide, 2 mils deep, spaced about mils apart, and have a rectangular cross section.

Referring to FIGURE 2 of the drawing, the wafer 20 is immersed in an atmosphere that will induce opposite type conductivity in the wafer. In this example, since the wafer is of N-conductivity type, the atmosphere selected is one which will induce P-conductivity type. A suitable P-type inducing material for silicon is boron. The wafer 20 is treated for about 3 minutes at about 1200 C. in a flowing ambient consisting of about 1 volume boron trichloride and 300 volumes nitrogen. Some boron trichloride decomposes and deposits boron in the Wafer surface. The ambient is then changed to pure nitrogen, and the Wafer is heated for about 4 hours at about 1300 C. The boron on the wafer surface diffuses into the wafer 20 during the heating, and forms a thin layer 22 of P-conductivity type over the entire wafer. Under these conditions, the P-type layer 22 is about 1.8 mils thick. A P-N junction 27 is formed at the interface of the P- type layer 22 and the N-type bulk of the wafer.

Referring to FIGURE 3 of the drawing, the major wafer surface opposite the grooved surface is then lapped so that the P-type layer 22 is removed. The N-type bulk of the wafer is thus exposed at this surface.

Referring to FIGURE 4, the wafer 29 is covered with a film 24 of material that induces the same conductivity type as the bulk of the crystal. In this example, a suitable material for inducing N-type conductivity in silicon is phosphorus. The Wafer 24) is heated for 20 minutes at 1200 C. in a stream of nitrogen which has been passed over phosphorus pentoxide kept at to C. An amorphous, glassy, phosphorus-containing film 24 is thereby formed over the entire wafer surface.

Referring to FIGURE 5, the unit is lapped again so' as to remove the phosphorus-containing film 24 from the lands only. The lapping is continued so as to remove about 0.2 mil of the silicon surface of the lands.

Referring to FIGURE 6, the unit is heated at 1300" C. in pure nitrogen for about two hours to diffuse the phosphorus from the remaining portion of the film 24 into the wafer. During this period the boron already present Will penetrate deeper into the water, so that the P-type layer 22 becomes thicker. introduced at the beginning of this step, of about 1200 C., for about 3 minutes, conductivity for the lands.

at a temperature to assure P-type The boron will only diffuse Some boron trichloride may be into the lands as the rest of the Wafer is covered by the phosphorus-containing film 24-. The phosphorus diifusion front travels faster than the boron, but does not catch up with it, so that a separation of. about 0.5 mil remains be tween the two fronts. The diffusion of the phosphorus causes the formation of an N-conductivity type zone 25 just below the surface of the wafer. Under these conditions, the N-type zone 25 is about 1.2 mils thick. A P-N junction 28 is formed at the interface of the N-type zone 28 and the P-type layer 22.

Referring to FIGURE 7, the phosphorus-containing surface film 24 is removed and the entire wafer surface cleaned before covering the wafer with a plating 26 of a 2 metal that makes a good ohmic contact but does not affect the conductivity type of the silicon. The metal plating 26 facilitates the fabrication of good electrical connections to the different regions of the completed device. A suitable metal for this purpose is nickel. nickel plating may be deposited over the surface of the silicon by the electroless nickel plating technique described by A. Brenner in Metal Finishing 52, No. 11, 68 (1954).

Referring to FIGURE 8, the unit is again lapped so as to widen the upper portion of each groove by 6 mils to a depth of about .7 mil. As the P-type zone 22 is about 1.8 mils thick, this lapping does not interfere with the continuity of the zone 22. The N-conductivity zone 25 and the heavily doped portion of the silicon is removed from the upper portion of each groove by this step. The removal of this material prevents shorting of the device by insuring a high reverse resistance between the emitter and base. The wafer is then diced into units by cutting along the lines a-a, bb, c-c, and along planes perpendicular to the length of the groove.

The unit thus formed is shown in FIGURE 9. Leads (not shown) are subsequently attached to the nickel plating on one land, on the groove bottom, and on the opposite surface. The N-type region on the groove bottom serves as the emitter. The P-type region under the groove bottom and under the lands serves as the base. The N-type bulk of the Wafer serves as the collector Transistors thus made exhibit a power gain as high as 40 decibels. This embodiment of the invention produces units with very thin and uniform base width, and is hence suitable for high frequency transistors.

Although the method has been described in terms of an N-conductivity type silicon wafer, it will be understood that the invention is equally applicable to other semiconductive, materials, such as germanium, cadmium telluride, gallium arsenide, and indium phosphide. For example,

the invention may be applied to a selenium doped N-conductivity type indium phosphide wafer. Regular grooves are lapped into one major surface, and the wafer is then heated in a cadmium atmosphere to form a thin P-conductivity type zone over the surface. A coating of tellurium is used to convert the grooved bottoms to N-conductivity type. Instead of nickel, a plating of indium is used to make ohmic contacts to the various regions of the device. It will be understood that the invention may be utilized beginning with P-conductivity type wafers by means of N- type-inducing ambient atmospheres.

Since the wafer is first lapped to form grooves, and is subsequently lapped again to widen the upper portion only of the grooves, a precise jig is necessary to insure that only the desired portions of the wafer are removed during each lapping operation. A suitable jig is shown in FIGURE 10. The jig consists of an accurate lap table 40 with a guide rail 41 running along the center line of the table sur.- face. The top surface of the guide rail 41 must be parallel to the top of the table 40. The height of the guide rail 46, in conjunction with the precise geometry of the different lapping heads, determines the depth of the grooves.

In operation, the silicon wafers are perferably embedded in a material such as paralfin wax and placed against the guide rail 41. The first type of lap-head 42 used is shown A bright adherent in FIGURE 11. It consists of a block of metal having a key way 43 that will just slide over the guide rail 41. The Working surface 44 of the laphead bears cutting grooves 45 at an angle of 30 degrees to the key way 43. Running the lap-head 42 back and forth over the guide rail 41 removes the desired amount of material from the upper surface of the wafer.

The second type of lap-head used is shown in FIGURE 12. This lap-head 52 also has a key way 53 that will just slide over the guide rail 41. The adjacent working surface 54 of the lap-head bears cutting bars 55 which are parallel to the key way 53, and cut the desired grooves into the wafers. The spacing and height of the bars determines the spacing and depth of the grooves. In order to widen the upper portion only of the grooves, a similar lap-head is used in which the bars are wider but not as high. In these two lap-heads the cutting bars must be identically spaced from the key way, so that the vcenters of the initial grooves and the secondary grooves coincide.

In the first embodiment of the invention, the emitter and the base are diffusion doped regions, while the collector is the original wafer. Another embodiment of the invention may be utilized in which the emitter and the collector are diffusion doped regions, while the base is the original wafer. Referring to FIGURE 13, a semiconductor wafer is prepared about 10 mils thick. The semiconductor may be silicon or germanium or one of the III-V compounds. In this example, the wafer is I- conductivity type silicon. Regular grooves 61 are lapped into one major surface of the wafer 60. The grooves 61 may be about mils wide, 6 mils deep, spaced about mils apart, and have a rectangular cross section.

Referring to FIGURE 14, the entire surface of the wafer 60 is covered with a phosphorus-containing film 64 by heating it for 20 minutes at 1200 C. in a stream of nitrogen which has been passed over phosphorus pentoxide kept at to C. The wafer 60 is then heated for about 3 hours at about 1300 C. in pure nitrogen, so that some of the phosphorus diffuses from the film 64 into the wafer and forms an N-conductivity zone 65 just below the surface of the wafer. The zone 65 thus produced is about 1.25 mils thick. A P-N junction 67 is formed at the interface of the N-type zone 65 and the P-type bulk of the Wafer.

Referring to FIGURE 15, the wafer 60 is lapped again so as to remove 2 mils of the material from the top of the lands. Since the N-type zone 65 is less than 2 mils thick, the original P-type bulk of the wafer is exposed at the lands. The wafer is then heated again for about 3 minutes at about 1200 C. in an atmosphere containing boron trichloride. This step deposits some boron on the lands, and is used to make certain that the lands are P-type. The rest of the wafer is not affected, since it is covered by the glassy phosphorus-containing film 64. The Wafer is next heated for about one hour at about 1300 C. in pure nitrogen. During this period the N-type region 65 becomes thicker, hence the P-type region between the grooves and the wafer bottom becomes thinner.

Referring to FIGURE 16, the phosphorus-containing film 64 is removed and the entire wafer surface cleaned before covering the wafer with an electroless nickel plating 66.

Referring to FIGURE 17, the wafer 60 is lapped again to Widen the upper 3 mils of the grooves 61. The nickel plating 66 and the N-conductivity zone 65 and the heavily doped port-ion of the silicon is thereby removed from the upper portion of each groove side. The removal of this material insures a high reverse resistance between emitter and base regions. The wafer is then diced into units by cutting along the lines aa, b-b, cc, and along planes parallel to the length of the wafer.

Leads (not shown) are subsequently attached to the nickel plating on one land, on the groove bottom, and on the opposite surface. In this embodiment the base region is the original wafer, hence the resistivity of the base may be more easily controlled. Another advantage of this embodiment is the high conductivity of the collector.

It will be understood that the heating cycles are mentioned by way of illustration, and not as limitations. Equivalent results may be obtained by heating at higher temperatures for shorter periods, or at lower temperatures for longer periods. Other doping materials may be used with appropriate changes in the processing procedure. The dimensions given are also by way of example only, and not as limitations, since wafers of any convenient size may be used. The thickness of the diffused zones in the wafers will depend on the materials used and the temperatures and duration of heating.

Other modifications may be made within the scope and spirit of the invention. For example, the recessed area prepared in one major wafer face may be circular flatbottomed pits. Such recessed areas may be made with a supersonic drilling tool, or with a conventional diamond drill. A larger drilling tool may be used to widen the upper portion of the recessed area. Other semiconductors may be utilized in place of silicon. For example, the starting material may be a wafer of indium-doped P-conduct-ivity type monocrystalline germanium. Grooves are lapped into one major wafer face. A surface layer of the wafer is then converted to N-conductivity type by heating it in an enclosed space, such as a well in a jig, together 'wtih a germanium source wafer containing sufficient N- type-inducing impurity to have a resistivity of about .001 ohm centimeter. Suitable impurity materials for this purpose are arsenic, antimony, and phosphorus. In this example, the source Wafer contains arsenic. The jig is heated in a hydrogen furnace for about minutes at about 800 C. Vapors of arsenic are given off by the source wafer, and fill the entire well. The vapors diffuse into the surface of the grooved .germanium wafer so that an N-conductivity type surface layer is formed. The thickness of the N-type layer is determined by the amount of impurity present, and the time and duration of heating. By accurately controlling the thickness of the diffused N- type zone, the step of heating the wafer in a P-type ambient such as boron trichloride to insure P-type lands may be omitted. An adherent conductive metal coating is deposited on the wafer .by the ame electroless nickel plating technique used with silicon. The remaining steps of lapping the Wafer to widen the upper portion of the grooves, dicing the wafer into units, mounting the units, attaching electrical leads, and encapsulating the units, are performed in the same manner as described above with reference to silicon.

There have thus been described new and useful forms of semiconductor devices as well as methods for making these devices.

What is claimed is:

1. A junction transistor comprising a given conductivity type monocrystalline semiconductive wafer bearing across one major face a groove between two lands,

the top of said groove being broader than the bottom,

said groove and lands having below the surface a zone of the opposite conductivity type for the base,

a thin surface layer of said given conductivity type on said groove bottom for the emitter,

a major face opposite said grooved face as the collector connection,

a metal plating over the top only of said lands and over the bottom only of said groove and on said wafer face opposite said grooved face,

said plating consisting of a metal which makes a good ohmic contact to said wafer but does not affect the conductivity type of said wafer,

and electrical leads to said metal plating on said lands, on said groove bottom, and on said opposite face.

2. A junction transistor comprising a given conductivity type monocrystalline semiconductive wafer bearing across one major face a groove between two lands,

the top of said groove being broader than the bottom,

said groove having below the sufrace a zone of opposite conductivity type for the emitter,

said landsbeing the base connection,

the Wafer face opposite said grooved face being the collector connection,

a metal plating over the top only of said lands and over the bottom only of said groove and over said water face opposite said grooved face,

said plating consisting of a metal which makes a good ohmic contact to said wafer but does not affect the conductivity type of said wafer,

and electrical leads to said metal plating on said lands, on said groove bottom, and on said opposite face.

3. A junction transistor comprising an N-conductivity 15 type monocrystalline silicon wafer bearing across one major face a groove between two lands,

the top of said groove being broader than the bottom,

said groove and lands having below the surface a P- .conductivity type zone for the base,

a .thin surface layer of N-conductivity type on said groove bottom for the emitter,

the wafer face opposite said groove face being the collector connection,

a metal plating over the top only of said lands and over the bottom only of said groove and over said wafer face opposite said grooved face,

said plating consisting of a metal which makes a good ohmic contact to said wafer but does not afiect the conductivity type of said wafer,

and electrical leads to said metal plating on said lands, on said groove bottom, and on said opposite face.

4. A junction transistor comprising a P-conductivity type monocrystalline germanium wafer bearing across one major face a groove between two lands,

the top of said groove being broader than the bottom,

conductivity type zone for the emitter,

a surface layer of N-cond-uctivity type on the face opposite said grooved face as the collector,

said lands being the base connection,

a metal plating over the top only of said lands and over the bottom only of said groove and over said wafer face opposite said grooved face,

said plating consisting of a metal which makes a good ohmic contact to said wafer but does not affect the conductivity type of said wafer,

and electrical leads to said metal plating on said lands, on said groove bottom, and on said opposite face.

5. A junction transistor comprising a monocrystalline phosphorus-containing N-conductivity type silicon wafer bearing across one major face a groove between two lands, the top of said groove being broader than the bottom, said groove and lands having below the surface a boroncontaining P-conduc-tivity zone for the base,

a phosphorus-containing N-conductivity type surface layer on said groove bottom for the emitter,

the wafer face opposite said grooved face being the collector connection,

a nickel plating over the top only of said lands and over the bottom only of said groove and on said wafer face opposite said grooved face,

said nickel plating making a good ohmic contact to said wafer but not affecting the conductivity type of said wafer,

and electrical leads to said nickel plating on said lands, on said groove bottom, and on said opposite face.

6. A junction transistor comprising a monocrystalline indium-containing P-conductivity type germanium wafer bearing across one major face a groove between two lands, the top of said groove being broader than the bottom, said groove bottom having below the surface an arseniccontaining N-conductivity type zone as the emitter, the face opposite said grooved face having an arseniccontaining N-conductivity type surface layer as the collector,

said groove bottom having below the surface an N-- V 7 said lands being the base connection, a nickel plating over the top only of saidlands and over the bottom only of said groove and on said wafer face opposite said grooved face,

' said nickel plating making a good ohmic contact to said wafer but not affecting the conductivity type of said wafer,

and electrical leads to said nickel plating on said lands,

on said groove bottom, and on said opposite face.

7. A junction transistor comprising a selenium-containing N-conductivity type monocrystalli-ne indium phosphide wafer bearing across one major face a groove between two lands,

the top of said groove being broader than the bottom,

said groove and lands having below the surface a cadmium-containing P-conductivity type zone for the base, 7

a tellurium-containing N-conductivity type surface layer on said groove bottom for the emitter,

said lands being the base connection,

an indium plating over the top only of said lands and A over the bottom only of said groove and on said opposite face,

said indium plating making an ohmic contact to said water,

and electrical leads to said indium plating on said lands, on said groove bottom, and on said opposite 5 face.

References Cited by the Examiner UNITED STATES PATENTS Wannlund et al. 317-235 FOREIGN PATENTS 12/54 Great Britain.

' DAVID J. GALVIN, Primary Examiner.

ARTHUR GAUSS, JAMES D, KALLAM, Examiners. 

1. A JUNCTION TRANSISTOR COMPRISING A GIVEN CONDUCTIVITY TYPE MONOCRYSTALLINE SEMICONDUCTIVE WAFER BEARING ACROSS ONE MAJOR FACE A GROOVE BETWEEN TWO LANDS, THE TOP OF SAID GROOVE BEING BROADER THAN THE BOTTOM, SAID GROOVE AND LANDS HAVING BELOW THE SURFACE A ZONE OF THE OPPOSITE CONDUCTIVITY TYPE FOR THE BASE, A THIN SURFACE LAYER OF SAID GIVEN CONDUCTIVITY TYPE ON SAID GROOVE BOTTOM FOR THE EMITTER, A MAJOR FACE OPPOSITE SAID GROOVED FACE AS THE COLLECTOR CONNECTION, A METAL PLATING OVER THE TOP ONLY OF SAID LANDS AND OVER THE BOTTOM ONLY OF SAID GROOVE AND ON SAID WAFER FACE OPPOSITE SAID GROOVED FACE, SAID PLATING CONSISTING OF A METAL WHICH MAKES A GOOD OHMIC CONTACT TO SAID WAFER BUT DOES NOT AFFECT THE CONDUCTIVITY TYPE OF SAID WAFER, AND ELECTRICAL LEADS TO SAID METAL PLATING ON SAID LANDS, ON SAID GROOVE BOTTOM, AND ON SAID OPPOSITE FACE. 